title: Evaluation of warfarin drug interaction listings in US product information for warfarin and interacting drugs.
abstract: BACKGROUND
  Because interactions with warfarin represent a serious risk to patients, drug information sources used by clinicians should contain accurate, timely, and practical drug interaction information.
  
  
  OBJECTIVE
  The aim of this study was to assess the information regarding warfarin interactions that is included in the official labeling of prescription products that interact with warfarin.
  
  
  METHODS
  We examined the official labeling information approved by the US Food and Drug Administration for the 50 drugs, biologics, and drug classes that were commonly identified by 3 drug information compendia--Clinical Pharmacology, ePocrates(®), and Micromedex(®)--and the warfarin US prescribing information (PI) as having an interaction with warfarin. The PI of each product was assessed for possible mention of an interaction with warfarin. The data were collected and tabulated by 1 investigator. A clinical investigator evaluated the data for accuracy and consistency. Unresolved issues were discussed with a third investigator and decided by consensus. The interaction listings were compared to determine similarities, differences, and inconsistencies and analyzed by 5 investigators.
  
  
  RESULTS
  Of the labeling for 73 products evaluated, 62 (85%) included mention of an interaction with warfarin. Those failing to mention the warfarin interaction were for older generic drugs or influenza vaccine. Among the labels listing an interaction with warfarin, the location of the information, the terminology used, and the inclusion of evidence for the interaction was inconsistent . When considering the PI for all 73 products, Fleiss' kappa coefficient (κ = 0.467) suggested moderate concordance according to the method of Landis and Koch.
  
  
  CONCLUSIONS
  This assessment of official US product labeling for 50 drugs, biologics, and drug classes known to interact with warfarin, comprising 73 distinct agents, found that 15% failed to mention the interaction, even though the interaction was mentioned in the warfarin labeling.
authors.affiliation: Center for Health Outcomes and PharmacoEconomic Research, The University of Arizona College of Pharmacy, Tucson, Arizona 85721-0202, USA.
authors.forename: Lisa E
authors.initials: LE
authors.lastname: Hines
authors.forename: Diana
authors.initials: D
authors.lastname: Ceron-Cabrera
authors.forename: Klaus
authors.initials: K
authors.lastname: Romero
authors.forename: Marietta
authors.initials: M
authors.lastname: Anthony
authors.forename: Raymond L
authors.initials: RL
authors.lastname: Woosley
authors.forename: Edward P
authors.initials: EP
authors.lastname: Armstrong
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Malone
mesh_terms: D000925:Anticoagulants / Q000009:adverse effects*; D004347:Drug Interactions; D004348:Drug Labeling / Q000592:standards*; D016568:Drugs, Generic / Q000009:adverse effects; D006801:Humans; D009626:Terminology as Topic; D014481:United States; D014486:United States Food and Drug Administration; D014859:Warfarin / Q000009:adverse effects*
chemical_list: D000925:Anticoagulants; D016568:Drugs, Generic; D014859:Warfarin